1. Field of the Invention
The present invention relates to telecommunications terminal blocks such as terminal blocks for connecting telephone service wires to telephone exchange distribution cables. More particularly, the present invention relates to providing electrical surge protection for telecommunications terminal blocks.
2. Description of Related Art
Telecommunications terminal blocks are used to provide convenient electrical connections between telephone customer service wires, or drop wires, (the "service" side) and telephone exchange distribution cables (the "exchange" side). Such terminal blocks typically connect up to 50 distribution cable wire pairs on the exchange side, which may have several thousand wire pairs, to up to 50 corresponding service wire pairs on the service side. Terminal blocks generally are configured as standard, multi-wire units which terminate either 3, 5, 10, 12, 15, 25 or 50 wire pairs.
The exchange side of the terminal block is connected to the exchange wires of the distribution cable through a stub cable. One end of the stub cable is typically connected to the exchange side of the terminal block within the terminal block. The other end of the stub cable is connected to selected wire pairs from the distribution cable. The permanent connection between the stub cable and the exchange side of the terminal block may be potted or provided within a chamber which seals the exchange side from the environment and provides a physically robust connection to withstand the recurring installing and removing of connections on the service side.
The service side of a terminal block is used to removably connect service wires to the distribution cable, through the permanent connection for the terminal block, so as to allow later disconnection and reconnection. Service wire pairs are typically connected to the terminal block through some type of terminal which is easy to connect and disconnect on-site such as a simple binding post where a stripped service wire is connected to the binding post and then secured with some type of cap. Another common type of terminal is an insulation displacement terminal where the service wire need not be bared prior to the connection to the terminal block and the insulation is severed through a blade or other sharp surface as the service wire is secured to the terminal. Again, in the insulation displacement type of terminal, some type of cap is typically employed to secure the service wire in place.
While the caps typically employed in the binding post or insulation displacement type terminals provide some protection from the environment, nonetheless, moisture, pollutants, chemicals, dust and even insects may reach the terminal connection resulting in corrosion or other degradation of the contact. This problem is exacerbated by the fact that in addition to the traditional aerial location of such terminal blocks, underground and even underwater terminal block locations are more and more frequently required for telephone distribution applications. Accordingly, efforts have been made to better insulate the terminal in the terminal block from the environment to prevent such degradation. One such approach has been to use a variety of insulating mediums, such as greases or gels to surround the terminal where the electrical connection is made.
Protecting telecommunications equipment against current and voltage surges is well known. Conventionally, the protection systems have been designed to resist major surges, e.g. due to lightning strikes or accidental connection to high voltage sources. Typically, protection provided for telecommunications lines is comparatively large and unwieldy, and therefore provided as a stand-alone package which is installed in concert with the lines to be protected. Due to their size, many of these systems are limited to protecting individual lines in areas without space restrictions such as telephone central offices or corporate offices which have adequate room to house individual protection for each line. Protection systems in this environment typically used gas tubes and, more recently, solid state devices to provide protection.
Increasingly, telecommunications terminal blocks connect service applications having sensitive electronic equipment, such as computers, directly to the telecommunication lines. As a result, protection against surges smaller than lightning strikes is needed. Such smaller surges may occur virtually anywhere along a system and hence more individualized protection for each line is needed.
Terminal blocks are available which provide protection in addition to terminating service wires to exchange wires. A prior art telecommunications terminal block, of the binding post variety, provides protection by providing a substantially larger terminal block which includes separate protection circuits. Each binding post which is used for service wire connections is connected to a corresponding screw-in type protector secured within a threaded protection retainer adapted to receive the screw-in protector. A protector may be added as needed to provide protection to a particular line or to permit replacement of a protector.
The prior art terminal block, as described above, is larger than a typical terminal block because it must provide the required room for the protection circuits. Moreover, as a binding post type terminal block, limited protection is provided against the environment. Due to the substantial space required and the limited protection against the environment, this prior art protected terminal block may be inadequate for installations where exposure to the environment can be expected or where terminal block space is limited.
Terminal blocks undergo extensive development and field testing prior to use in the field to ensure a particular design is capable of withstanding the difficult environmental and operational challenges inherent in terminal block use. As a result, users tend to be faithful to terminal block designs which have proven themselves rugged and reliable over time. With the advent of an increasing need for protection in terminal blocks, it would be desirable to be able to add protection using existing terminal block designs without requiring extensive redesign. Extensive redesign requires additional testing, new tooling and, in the mind of the user, could call into question the terminal block's environmental or operational integrity.
Another issue which is raised in providing protection is related to how often line protection is needed. In some applications every line connected to a particular terminal block may need to be protected. Protecting a specific line can be costly, however, due to the components involved in providing protection. Therefore, in some cases a particular user may decide that certain lines do not require protection or are not worth the cost of protection in view of the probability that a voltage surge may occur. As a result, it is desirable that a protected terminal block be provided in which the user has the option to determine which lines may need protection and add protection to those lines. Further, it is desirable to permit the user to either add or remove the protection, as the used of the lines involved changes.
For the foregoing reasons, there is a need for an improved telecommunications terminal block having protection against electrical surges.